[6.1] Vector ref assembly fixes and enable vector and json tests

[apoorvdeshmukh]

Description

Ports #3521 and enable vector and json test coverage through port of #3559.

Testing

Adds a testcase for validation of ref assembly APIs for SqlVector.

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[Copilot]

Pull Request Overview

This PR ports vector-related API fixes and enables test coverage for vector and JSON functionality by making them available on Azure SQL. The changes update reference assemblies to fix the nullable return type for SqlVector<T>.Null and enable previously disabled vector and JSON tests.

• Fixes the return type of SqlVector<T>.Null property from non-nullable to nullable in reference assemblies
• Enables vector and JSON test support by updating feature flags to be based on Azure SQL detection
• Adds a new validation test for SqlVector API functionality in manual tests

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/VectorTest/VectorAPIValidationTest.cs	New test file for validating SqlVector reference assembly APIs
src/Microsoft.Data.SqlClient/tests/ManualTests/Microsoft.Data.SqlClient.ManualTesting.Tests.csproj	Reorganizes test file inclusions, moving vector and JSON tests from excluded to included
src/Microsoft.Data.SqlClient/tests/ManualTests/DataCommon/DataTestUtility.cs	Updates feature flags to enable JSON and Vector support for Azure SQL
src/Microsoft.Data.SqlClient/netfx/ref/Microsoft.Data.SqlClient.cs	Fixes SqlVector Null property return type to be nullable
src/Microsoft.Data.SqlClient/netcore/ref/Microsoft.Data.SqlClient.cs	Fixes SqlVector Null property return type to be nullable

[mdaigle]

Nit: Sharing test data across classes like this can make it hard to update tests in the future. It's fine to replicate simple test data. This method could also be broken out into individual tests for the float[] constructor, ROM constructor, null vector, and create null method.

[cheenamalhotra]

@apoorvdeshmukh please address this in 'main'. I will proceed with this PR in release/6.1 branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.04%. Comparing base (7a7b54e) to head (8cb5f5f).
⚠️ Report is 3 commits behind head on release/6.1.

❗ There is a different number of reports uploaded between BASE (7a7b54e) and HEAD (8cb5f5f). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (7a7b54e)	HEAD (8cb5f5f)
addons	1	0

Additional details and impacted files

@@               Coverage Diff               @@
##           release/6.1    #3562      +/-   ##
===============================================
- Coverage        69.69%   61.04%   -8.66%     
===============================================
  Files              281      275       -6     
  Lines            62413    62099     -314     
===============================================
- Hits             43500    37907    -5593     
- Misses           18913    24192    +5279     

Flag	Coverage Δ
addons	?
netcore	63.35% <ø> (-9.42%)	⬇️
netfx	63.60% <ø> (-5.64%)	⬇️

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